Copper and copper alloys are widely used in the field of semiconductor fabrication as conducting materials. As a conductor, copper is often preferred to other metals, such as aluminum, due to its high electrical conductivity and good electromigration resistance properties. Because of these advantages, copper-filled lines and vias are now ubiquitously seen as conducting paths connecting elements of semiconductor devices, such as in integrated circuits.
Processing of copper during fabrication of semiconductor devices, however, presents a set of challenges. Because copper is not easily amenable to plasma etching, copper-containing devices typically need to be fabricated using damascene processing. In damascene processing, copper is deposited as an inlay on a substrate having a pattern of pre-formed recessed damascene features, such as vias and trenches. The pattern of recessed features is usually formed by photolithographic techniques. After the recessed features have been formed, copper is globally deposited onto the substrate such that it fills the recessed features and also forms an overburden layer over the field region, wherein the field region refers to the top plane of the substrate prior to copper deposition. Subsequently, the overburden is removed by a planarization technique such as chemical mechanical polishing (CMP), providing a planarized substrate having a pattern of copper-filled conductive paths.
While methods for efficient copper removal are desirable at various stages of semiconductor device fabrication, conventional wet copper etching techniques have not been widely introduced because they generally could not be successfully integrated into semiconductor device fabrication processes. One of significant drawbacks of conventional etching chemistries includes their anisotropic nature. Anisotropic etching leads to preferential etching of copper in one specific direction and/or preferential etching of one type of grain orientation and, consequently, leads to roughening of copper surface, pitting, and grain boundary dependent non-uniform copper removal. Furthermore, anisotropic etch rates are typically higher on those surfaces that are more exposed to the bulk etching solution. For example, with conventional anisotropic etching, edges of an isolated feature, or features at edges of pattern array will etch with different properties and rates compared to those areas that are more removed from (and less exposed to) the bulk of the etching solution. This drawback, in many instances, cannot be tolerated in semiconductor fabrication where clean, smooth, and isotropic removal of copper is typically desired. Common examples of known acidic copper etches that are anisotropic include: a nitric acid etch, where nitric acid is used typically alone and acidic mixtures containing an oxidizing agent, such as hydrogen peroxide, permanganate, ferric ion, bromine, and chromium (VI) and an acid (e.g., acetic or sulfuric). Common examples of known neutral and alkaline etches that are anisotropic include: ammonium or alkali metal persulfate solutions, ferric chloride based solutions, and ammonium hydroxide based solutions.